This invention relates to electrodes for use in batteries, and in related applications. The invention also relates to battery cells.
The commonly employed Ni/Cd secondary battery cell has a number of important disadvantages. The open cell voltage (typically around 1.15 volts) is too low for the battery to be conveniently interchangeable with primary batteries having an open cell voltage of around 1.5 volts. Both forms of cadmium appearing in the battery--metallic cadmium and cadmium oxide--are known to be toxic and the standard electrolyte of concentrated potassium hydroxide is extremely corrosive. For these reasons, guaranteed containment of the cell contents is essential and the production costs of the cell housing are relatively high.
The number of charge/discharge cycles that can reliably be undergone by a Ni/Cd battery may be enough for some domestic purposes but is not sufficient for many specialised applications where a battery is required to undergo reliably 1,000; 5,000 or even more charge/discharge cycles. An important factor in limiting the charge/discharge life of a Ni/Cd battery is the internal stress created upon the change of phase from metallic cadmium to cadmium oxide. Each charge/discharge cycle involves a structural change within the cadmium electrode resulting, over a few hundred cycles, in mechanical deterioration. The same factor will apply to a number of other known battery electrodes such as MnO.sub.2, which involve a change of phase.
In cases where there is a limited supply of charging energy, it is necessary to take into account the energy associated with the phase change. When used, for example, as a storage battery in combination with solar cells, the Ni/Cd battery is able to make less use of the solar charging energy than would be the case if no phase change were involved.
It should further be noted that cadmium oxide, in common with a number of other known electrode materials, is insufficiently electrically conductive to be used directly as an electrode and requires to be mixed with graphite in production of a battery cell. This may involve manufacturing difficulties. The need to incorporate the material within an electrode which is not inherently electroactive is more generally regarded as an important drawback.
Reference is directed to Journal of the Electrochemical Society July 1987, pages 1591-1594 Hashemi et al "Dicadmium Stannate as a Novel Electrode Material for Battery Applications". This proposes an electrode for secondary battery cells which offers some advantages over the conventional electrodes.
It is one object of the present invention to provide a further improved electrode which can be used in a secondary battery cell substantially to overcome some or all of the described disadvantages of known secondary battery cells.